The present invention relates to a method and apparatus for thermal chemical reactions. More specifically, the method and apparatus provide an enhanced reaction rate for the thermal chemical reaction.
As used herein, the term thermal chemical reaction(s) includes exothermic and endothermic chemical reactions.
Thermal chemical reactions including exothermic and endothermic chemical reactions are well known. Examples of thermal chemical reactions include but are not limited to Hydrogen and Hydrocarbon conversion reactions including but not limited to steam reforming, water-gas shift reactions and combustion are well known. These reactions are usually carried out in the presence of a catalyst at temperatures up to about 1000xc2x0 C. Because the intrinsic kinetics of the thermal chemical reaction are much faster than the heat transfer rate between the reaction vessel and the thermal sink or environment, the rate of product production is limited. Limited production rates may be characterized in terms of residence time which is typically seconds to minutes in convention thermal chemical reaction vessels.
For example, the water gas shift reaction is conventionally carried out in fixed bed reactors. The water gas shift reaction of converting carbon monoxide and water to carbon dioxide and hydrogen suffers from multiple-second residence times (kinetic impediment) when carried out in fixed bed reactors. Theoretical kinetics suggests possible residence times on the order of milliseconds. There are two kinetic retarding aspects to conventional reactors. The first is a diffusion limitation as reactants diffuse into and out of a catalyst bearing porous pellet and the second is a heat transfer limitation which is a combination of heat transfer parameters (conduction, length) of catalyst supports and overall reactor geometry (shape and size). Because the water gas shift reaction is critical to a multi-reactor fuel processing system that supports distributed energy production through the use of a fuel cell, there is a need for a smaller, faster water gas shift reactor.
Another example is conventional methane steam reforming reactor produces synthesis gas at an average residence time of several seconds and with an effectiveness factor of 0.01 to 0.05 reported by Adris, A., Pruden, B., Lim, C., J. Grace, 1996, On the reported attempts to radically improve the performance of the steam methane reforming reactor, Canadian Journal of Chemical Engineering, 74, 177-186. In typical industrial operation, the methane to steam ratio is run at 3:1 to prevent coke formation.
Efforts to improve heat transfer between the reaction vessel and the thermal sink have made only modest improvements in product production rate. Thus, there is a need in the art of thermal chemical reactions for a method and apparatus that increases the heat transfer rate between the reaction vessel and the thermal sink and thereby approach the theoretical intrisic kinetic rate of reaction and production.
The present invention is a method and apparatus for obtaining an enhanced production rate per reaction chamber volume of a reaction chamber with an inlet and an outlet for a thermal chemical reaction, wherein a ratio of the enhanced production rate per reaction chamber volume to a conventional production rate per conventional reaction chamber volume for the thermal chemical reaction is at least 2. For example, for convention steam reforming residence time is on the order of seconds whereas with the present invention, residence time is less than a factor of 2, on the order of milliseconds. The method and apparatus rely upon;
(a) a porous insert within the reaction chamber volume, wherein a reactant flow substantially completely passes through the porous insert wherein the reaction chamber volume with the porous insert has a mean porosity less than 1 and a transport distance no greater than 3 mm;
(b) the reaction chamber volume with a length parallel to a bulk reactant flow, the length less than or equal to 6 inches, and with a height less than or equal to 2 inches, thereby transferring reaction heat at an enhanced heat transfer rate through the porous insert; and
(c) a heat transfer chamber in thermal contact with the reaction chamber volume, the heat transfer chamber transferring heat at said enhanced heat transfer rate across a wall between the heat transfer chamber and the reaction chamber, thereby obtaining the enhanced production rate per reaction chamber volume for the thermal chemical reaction wherein a ratio of the enhanced production rate per reaction chamber volume to a conventional production rate per conventional reaction chamber volume for the thermal chemical reaction is at least 2.
These features have been found to cooperate with the reaction kinetics in terms of transferring heat at a rate sufficient to avoid substantial impediment of the kinetics. These features are effective for both catalytic and non-catalytic thermal chemical reactions. For catalytic chemical reactions, addition of a catalyst upon the porous insert permits flow of reactants past catalyst sites rather than limiting reactant motion to diffusion as in conventional systems. Thus, according to the present invention, for catalytic thermal chemical reactions, both kinetic impediments are substantially reduced permitting realization of theoretical or near theoretical reaction kinetics. More specifically, a water gas shift reactor made according to the present invention has {fraction (1/10)}th to {fraction (1/100)}th the size of conventional processing hardware for the same production output.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.